1. Field of the Invention
The present disclosure relates to an image forming apparatus, and more specifically relates to an image forming apparatus compatible with a high speed apparatus.
2. Related Art
In recent image forming apparatuses, space-saving vertical transport-type image forming apparatuses have become mainstream in order to reduce installation space. For example, a plurality of sheet feed portions (i.e., paper feed trays or paper feed cassettes) are arranged at multiple levels in the lower portion of the apparatus main body, and an image forming portion and a fixing portion are arranged thereabove. The image forming portion forms an image on a recording sheet such as paper at an image forming region that is disposed on a sheet transport path for transporting a recording sheet, and the fixing portion fixes the image formed by the image forming portion.
FIGS. 27A and 27B are explanatory views for illustrating an exemplary configuration for forming an image on paper P at an image forming region (i.e., a transfer nip portion N1) that is disposed on a sheet transport path 228 for transporting the paper P. FIG. 27A is a schematic side view showing the configuration thereof, and FIG. 27B is a schematic side view showing an enlarged view of registration rollers R51 and R52 that are arranged on the upstream side of an image forming region (i.e., the transfer nip portion N1) in a transport direction Y1 of the paper P.
In the configuration shown in FIG. 27A, the paper P fed from a sheet feed portion (not shown) is once transported upward and then transported toward the transfer nip portion N1.
On the sheet transport path 228, a pair of transport rollers R31 and R32, pre-registration rollers R41 and R42, and the registration rollers R51 and R52 are arranged in this order in the transport direction Y1 of the paper P. The registration rollers R51 and R52 face the transfer nip portion N1 at a predetermined distance (e.g., distance of approximately 50 mm) therefrom. The transfer nip portion N1 is a portion in which an intermediate transfer member or an image bearing member (a photosensitive drum 214 in the drawing) and a transfer roller 217a are in contact with each other.
FIG. 28 is a timing chart showing the operation timings of the registration rollers R51 and R52 and the pre-registration rollers R41 and R42.
As shown in FIGS. 27A, 27B, and 28, when the paper P is transported through the sheet transport path 228 up to the registration rollers R51 and R52, the registration rollers R51 and R52 stop first at a time t1, and, when a leading edge (edge on the downstream side in the transport direction Y1) of the paper P makes contact with a nip portion N5 between the registration rollers R51 and R52, the pre-registration rollers R41 and R42 stop at a time t2 that is slightly after the time t1. Due to stoppage according to this time difference, the paper P sandwiched by the registration rollers R51 and R52 and the pre-registration rollers R41 and R42 is kept bowed in a slight curve. When the paper P is slightly bowed in this manner, tilting of the paper P is prevented, and distortion of a formed image with respect to the width direction of the paper P is eliminated. That is to say, regarding the paper P temporarily stopped by the registration rollers R51 and R52, the sheet transport state such as tilting of the paper P during transport is corrected by the registration rollers R51 and R52.
Subsequently, the registration rollers R51 and R52 and the pre-registration rollers R41 and R42 start transport of the paper P again at a time t3 where an image forming position at which an image is to be formed on the paper P is caused to match an image writing position (e.g., leading edge) of image information made visible on the photosensitive drum 214 in the transport direction Y1 (sub-scanning direction) (see FIG. 27A). Accordingly, the image forming position on the paper P from the registration rollers R51 and R52 can be caused to match the image writing position onto the photosensitive drum 214 in the transport direction Y1 (sub-scanning direction). At that time, writing of the image information is performed onto the photosensitive drum 214 such that the image forming position on the paper P matches the image writing position onto the photosensitive drum 214 also in the width direction (main-scanning direction) along a recording sheet face orthogonal to the transport direction Y1 of the paper P.
Incidentally, there is an increasing demand for recent image forming apparatuses to perform an image forming (printing) process at higher speed. For example, conventionally, an image forming apparatus capable of 60 sheets per minute (in the case of A4 sideways transport) or more was deemed to be a high speed apparatus, but recently, an image forming apparatus capable of 80 sheets per minute or more is referred to as a high speed apparatus, and moreover, development of image forming apparatuses capable of 100 to 120 sheets per minute or more is progressing. Thus, the processing speed of image formation in color printing is also increased so as to be more than 70 sheets per minute.
In this sort of image forming apparatus, it is necessary to maintain or improve the image quality when transferring a toner image on a photosensitive drum onto the paper P. An important factor for maintaining or improving the image quality is maintaining or improving the precision of the positional matching of the image writing position onto an image bearing member and the image forming position on a recording sheet.
As a technique regarding the precision of positional matching, an image forming apparatus is proposed in JP 2003-330334A.
JP 2003-330334A discloses an image forming apparatus that detects a length of displacement in a direction orthogonal to the recording sheet transport direction, and performs control so as to continue image formation if that displacement length does not exceed a prescribed value, and temporarily stop recording sheet transport if that displacement length exceeds the prescribed value.
However, due to the recent demand for further increasing the processing speed of image formation, during successive printing of a plurality of sheets, it is difficult to perform timing adjustment of recording sheets transported subsequent to a first recording sheet. That is to say, when printing the first sheet, there is time to spare in the initialization process or the like of the image forming apparatus, and, thus, it is possible to secure time to spare for adjusting the image writing position (image writing position in the transport direction and/or the width direction) by making earlier the timing of feeding a recording sheet from a sheet feed portion. However, the timing of transporting the second and subsequent recording sheets depends on the print processing speed, that is, the transport speed, and writing of image information onto the image bearing member has started before the leading edge of the second and subsequent recording sheets makes contact with the nip portion between the registration rollers, and, thus, no time to spare is available for adjusting the desired image writing position at which an image is to be formed on the recording sheet.
Regarding this aspect, Japanese Patent No. 4315988 discloses an image forming apparatus in which other recording sheets, on which image formation is to be performed after a preset number of recording sheets, are subjected to image formation at an image forming region based on a corrected image writing position, that is, detection by a sheet transport position detecting portion regarding the preset number of recording sheets is used regarding the image forming position of the other recording sheets, thereby enabling image formation to be performed while correcting the image writing position of a plurality of recording sheets even in a high speed apparatus.
However, in the image forming apparatus disclosed in Japanese Patent No. 4315988, since detection by the sheet transport position detecting portion regarding the preset number of recording sheets is used regarding the image forming position of recording sheets other than the preset number of recording sheets, for example, if the transport rollers are expanded by heat generated by friction or the like or recording sheets are replenished to sheet feed portions such as paper feeds tray or paper feed cassettes during the process of the preset number of recording sheets, so that the sheet transport position (position in the transport direction and/or the width direction) is suddenly significantly displaced, the image writing position based on the used detection is significantly displaced from the proper image writing position (based on an actually detected sheet transport position). Therefore, the precision of positional matching of the image forming position on a recording sheet and the image writing position onto an image bearing member becomes poor.